1. Field of the Invention
The present invention relates to a harmonic suppression circuit for attenuating harmonics developed in an amplifier, etc.
2. Description of the Related Art
A conventional harmonic suppression circuit will be described using FIGS. 3 through 6. FIG. 3 shows a circuit for suppressing a multiple harmonic, and FIG. 5 illustrates a circuit for suppressing a multiple harmonic and a triple harmonic simultaneously, respectively. FIGS. 4 and 6 respectively show characteristics of the harmonic suppression circuits shown in FIGS. 3 and 5.
Referring first to FIG. 3, an amplifier 11 is a high-frequency amplifier such as a receiver or the like. The amplifier 11 is configured or formed on an unillustrated printed board and amplifies signals lying within an SHF band (e.g., 5.8 GHz). The amplifier 11 has a high amplification factor to amplify a weak signal and generates unnecessary harmonics (multiple harmonic and triple harmonic in particular) at a high level. Therefore, a microstrip line 12 for attenuating the harmonics is provided on the output side of the amplifier 11. The microstrip line 12 is formed on the printed board. One end of the microstrip line 12 is electrically connected to a signal transmission line 13 provided on the output side of the amplifier 11, and a terminal thereof is closed. Further, the length of the microstrip line 12 is set to 1/8 of the wavelength with respect to a frequency (corresponding to the frequency of a fundamental wave) of a received signal. Therefore, the impedance on the microstrip line 12 side as viewed from the signal transmission line 13 becomes extremely small at a frequency (multiple harmonic, frequency: 2f.sub.0) corresponding to twice the frequency of the received signal. Thus, the multiple harmonic (2f.sub.0) is attenuated as shown in FIG. 4.
When it is desired to attenuate a triple harmonic (3f.sub.0) as well as the multiple harmonic (2F.sub.0), another microstrip line 14 is further electrically connected to the signal transmission line 13 as shown in FIG. 5 and the termination of the microstrip line 14 is opened. The length of the microstrip line 14 is set to 1/12 of the wavelength with respect to the frequency (f.sub.0) of the received signal. As a result, the impedance on the microstrip line 14 side as viewed from the signal transmission line 13 becomes extremely small with respect to the harmonic (3f.sub.0) corresponding to three times the frequency of the received signal (theoretical impedance is 0 ohm). Thus, the triple harmonic (3f.sub.0) is attenuated as shown in FIG. 6. Further, the multiple harmonic (2f.sub.0) and the triple harmonic (3f.sub.0) can be attenuated simultaneously.
In the above-described conventional construction, however, the microstrip line 12 for attenuating the multiple harmonic (2f.sub.0) reduces the impedance at the frequency (f.sub.0) of the fundamental wave and attenuates the signal of the fundamental wave (f.sub.0) to 5 to 8 dB as is apparent from points A and B shown in FIG. 4 and 6. Therefore, a problem arises in that the received signal becomes short of sensitivity, the circuit connected to the stage subsequent to the high-frequency amplifier 11 is impeded in operation, and NF is deteriorated.